MESOZOIC GEOLOGY

Overviews

• Breakup of Pangea

  • Began in the Permian and was completed in the Cenozoic (Eocene).

  • Dominated the geological events of the Mesozoic.

• Triassic

  • Laurasia (including North America) began to separate from Gondwana as rifting began to create the North Atlantic Ocean.

  • The rifting produced tensional geologic features both in eastern North America and western Africa.

  • The western margin of North America became an active continental margin with subduction, volcanism, and orogeny as the continent drifted relatively westward and collided with oceanic crust. Sonoma orogeny.

  • Accreted terranes began to be added to the western margin of North America.

• Jurassic

  • Rifting and the breakup of Pangea continued.

  • Africa, Antarctica, and India separated.

  • South America began to split away from Africa.

  • Pulses of subduction, volcanism, orogeny, and accretion continued in the Cordilleran region. Nevadan orogeny.

• Cretaceous

  • South America and Africa completely separated by end of Cretaceous; South Atlantic Ocean created.

  • More pulses of subduction, volcanism, orogeny, and accretion in the west. Sevier orogeny. Laramide orogeny began.

Triassic Period

• Separation of North America (Laurasia) from Africa

  • The Alleghenian mountains had been worn down to a plain by erosion.

  • Tensional forces arose as Laurasia separated from Africa, forming the ancestral North Atlantic Ocean. "Fault-block" mountain ranges separated by basins were created. The mountains were blocks of crust that rose, called horsts, and the basins were blocks that subsided, called grabens. Both were bordered by normal faults (indicating tension).

  • The basins filled with continental sediment. Because of the dry climate and rapidity of sedimentation, the sediments were mostly immature (contained incompletely weathered minerals) and poorly sorted. Common types are arkose (sandstone containing abundant feldspar) and other types of sandstones and conglomerates, derived from granite.

  • The tensional forces thinned the crust and allowed magma to rise to the surface, leading to widespread volcanism.

• Formation of the Gulf of Mexico

  • The region that was to become the Gulf of Mexico was a broad evaporation basin in the late Triassic.

  • Evaporites, including salt, began to accumulate to great thicknesses.

• Continental Interior

  • The Triassic lacked an epeiric sea, resulting in alluvial (river and stream) deposits in the interior of the continent. Many of these were redbeds (sandstones and sandy shales cemented by iron minerals which, when oxidized , give them a reddish color).

• Cordilleran

  • The western motion of North America resulted in a subduction zone and island arcs along the western margin of the continent as an oceanic plate was subducted beneath the continent beginning in the early Triassic.

  • This mountain-building episode is called the Sonoma orogeny.

  • The orogeny involved both thrust faulting and volcanism.

  • Oceanic rock and marine sediments were thrust over the roots of the Middle Paleozoic Antler orogeny.

  • Volcanic rocks and graywackes (a type of sandstone containing abundant rock fragments) were deposited to the west associated with an island arc there.

  • To the east sandstones from erosion of the Sonoma highlands were interbedded with the continental redbeds mentioned previously. Where sea existed and sedimentation was low, limestones formed.

Jurassic Period

• Eastern Continental Margin Became Passive

  • Rifting continued as the North Atlantic widened.

  • Tectonic activity (fault-block mountain formation) died down as the eastern margin got farther and farther from the plate boundary.

  • A typical passive continental margin, with continental shelf, continental slope, and continental rise, began to form on the eastern margin from sediment derived from the previously formed mountains.

• Gulf of Mexico

  • After over one kilometer of evaporites accumulated, deeper sea water began to invade the young Gulf.

  • The Gulf experienced alternating transgressions and regressions of the sea.

  • Several kilometers of various sediments (limestones, limy muds, sandstones) were deposited, depending on the depth of the sea and distance from sediment sources.

• Advance and Retreat of the Sundance Sea

  • The Sundance epeiric sea advanced from Canada to the Gulf of Mexico by the Middle Jurassic.

  • Apparently there were abundant coastal sand dunes on the western margin of the sea (in the general area of southern Utah). The famous massively cross-bedded Navajo sandstone probably derives from these dunes.

  • As the sea retreated it left river and stream deposits that formed the Morrison Formation, famous for its dinosaur fossils. Locations with broken-up dinosaur remains were probably due to occasional severe flooding events.

  • The retreat of the sea was likely due to the developing Nevadan orogeny, which produced a range of mountains extending from Idaho to Arizona and New Mexico.

• Nevadan Orogeny

  • The Nevadan orogeny was larger and more intense than the Sonoma of the Triassic.

  • It evolved from two parallel subduction zones off the coast of western North America, the western one dipping west and the eastern one dipping east. By the Late Jurassic there was just one subduction zone dipping east and the Nevadan orogeny began.

  • The orogeny migrated eastward with time, possibly due to a change from a more steeply to a less steeply dipping subduction zone.

  • It involved volcanism, severe folding of sediments, thrust faulting, and the emplacement of huge granitic batholiths in zones elongated in a north-south direction. These batholiths were probably the roots of volcanic mountain ranges, possibly like the situation with the Andes in western South America today where there is a subduction zone along the western margin of that continent.

Cretaceous Period

• Eastern Passive Continental Margin Matures

  • The continental shelf, slope, and rise became well developed.

  • The Appalachian area experienced renewed uplift, providing thick sediments for the coastal plain as it subsided.

  • Meanwhile, Florida was mostly a submarine bank on which limestone was deposited.

• Zuni Epeiric Sea

  • The Zuni sea advanced in the Early Cretaceous, regressed in the Middle Cretaceous, and advanced again in the Late Cretaceous.

  • At its greatest extent it reached from the Gulf to the Arctic and from Nevada to the southeast U.S.

  • The sediments deposited in the sea were clastics and limestone (including chalk).

  • The Cretaceous is famous for the worldwide deposition of chalk, made up of the tiny carbonate shells ("tests", called coccoliths) made by a form of algae. Even today similar species often reproduce rapidly to form "blooms".

  • The reason for the vast seas of the Cretaceous could be due to rapid sea-floor spreading then. Rapid sea-floor spreading would result in a warmer than average ocean crust, which, being less dense due to its higher temperature, would tend to rise and displace the ocean onto the land.

• Nevadan and Sevier Orogenies

  • The Sierra Nevada batholith complex was emplaced in the Early Cretaceous.

  • The Sevier orogeny overlapped with the Nevadan and was not as "severe".

  • It is thought to have been caused by low-angle subduction, producing thrust faulting to the east of that due to the Nevadan orogeny.

  • The Sevier orogeny is noted for the low-angle, imbricated thrust faults associated with it.

• Laramide Orogeny

  • This orogeny, which began in the Late Cretaceous, was of a different nature than the previous orogenies in that it involved the formation of anticlines and domes, basins, monoclines, with much less thrusting than previous orogenies.

  • It affected the Cordilleran from Alaska to Texas into Mexico and resulted in a broad uplift of the Cordilleran.

  • One explanation for the Laramide is that the orogeny resulted from the crust reacting to vertical movements along deep faults in the Precambrian basement rocks. This fault movement, in turn, may have been due to a shallow subduction slab scraping the bottom of the continent.

  • The Laramide orogeny continued well into the Cenozoic Era.

• Accretion of Terranes

  • "Terranes" (also called "exotic terranes", "suspect terranes", and "accreted terranes") are regions of rock that have similar characteristics internally but differ markedly from surrounding rocks.

  • The terranes of the Cordilleran are thought to be foreign to North America, because they have different fossils, different stratigraphy, and different paleomagnetic properties to rocks formed in place.

  • They may be continental fragments, island arcs, etc., that collided with the western edge of the continent due to plate motion.

  • Estimates vary, but 50 identified terranes may make up as much as 70% of the Cordilleran. (Note this estimate differs from that in the textbook.)